The core of a typical nuclear reactor may include 40,000 or more fuel rods, each containing a column of hundreds of fuel pellets. The fuel rods are arranged to generate controlled amounts of heat in specific regions of the core. Controlled heat generation is largely achieved by organizing the pellet columns in specified zones of varying lengths and uranium enrichment concentrations. Current designs call for fuel rods having as many as seven pellet zones of various specific lengths and four or more different enrichment concentrations.
Proper operation of a reactor critically depends not only on the locations of the various types of fuel rods within the core, but also on the locations of the various pellet zones of specified enrichments in the pellet column of each fuel rod. Thus, to assure requisite control of the heat generated in a reactor core such as to decrease local power peaking, to improve the power distribution throughout the bundles of fuel rods, and to provide adequate reactor shutdown margin, it is critical that the fuel rods be manufactured strictly in accordance with engineering specifications to comply with safety and regulatory requirements. Thus, each and every fuel pellet must be in a prescribed location within the pellet column according to enrichment concentration in order to achieve requisite enrichment zone lengths and zone interface positions along the column length.
As the number of enrichment zones per pellet column increases, so does the potential for manufacturing error. Thus more rigorous quality assurance measures must be instituted. Pellets of different enrichments must be maintained segregated, tracked and accounted for throughout the loading process. Each pellet enrichment zone must be precisely made up to exacting length and weight specifications and loaded into a fuel rod or cladding tube in the proper order to assure its requisite positioning in the pellet column. That is, the fuel rod must be loaded by pellet zones in accordance with predetermined engineering specifications or so-called "rod maps". Another quality assurance check is the vacant space left in the cladding tube after the last pellet zone has been loaded, which ultimately provides a plenum chamber when the tube is sealed with an end plug as the final fuel rod manufacturing step. A record must be kept of the specifications to which each rod was loaded with pellets so that the rods can be assembled into fuel bundles in proper positions.